On any given day, more than a thousand suspects in criminal investigations will voluntarily take polygraph examinations in the hope of being cleared. The analyses performed are based on the assumption that, when deception is attempted, small changes in human physiology occur as a result of either cognitive processing or emotional stress.
Polygraph tests are administered by more than two thousand trained and experienced examiners in the United States, Canada, Japan India, Israel, Saudi Arabia, Turkey, and many other nations. Somewhere between 40% and 60% of those who take the tests will be cleared on the basis of an examiner's decision of "No deception indicated." For those who are not cleared, the criminal investigative process will continue. Polygraphs affect the lives of many people, from those who are the victims of criminals to those who are suspects.
While the primary use of the polygraph test is during the investigative stage of the criminal justice process, polygraph results are sometimes presented in court as evidence. Polygraph tests also play a small role in parole and probation supervision.
In addition to the significant role in criminal justice, polygraph examinations are also used for national security, intelligence, and counterintelligence activities of the United States and foreign nations. Thousands of federal screening examinations are used annually to grant or deny clearance and access to sensitive operations and material.
A polygraph test format is an ordered combination of relevant questions about an issue, control questions that provide physiological responses for comparison, and irrelevant (or neutral) questions that also provide responses or the lack of responses for comparison, or act as a buffer. All questions asked during a polygraph test are reviewed and discussed with the examinee and reworded when necessary to assure understanding, accommodate partial admissions, and present a dichotomy answerable with a definite "yes" or "no." During the test, the questions are delivered in a monotone voice to avoid emphasis on one question or another.
Polygraph examiners have a choice of several standard test formats. The examiner's decision on which format to use will be based on test objectives, experience, and training.
Three classes of test formats are used: control question tests, concealed knowledge tests, and relevant-irrelevant tests. Each format consists of a prescribed series of questions that together make up a chart. Two to five charts make up a test.
The majority of criminal investigation tests are conducted by using one of the possible formats of control question tests. These tests consist of a series of control, relevant, and irrelevant questions. Each question series is repeated two to five times, and each series produces a separate chart.
An example of a relevant question is, Did you embezzle any of the missing $12,000?. For this test format, the corresponding control questions will be about stealing; the questions are threatening to the subject but are not about the theft at issue. An example is, Before you were employed at this bank, did you ever steal money or property from an employer? The control and relevant questions will be compared.
Irrelevant questions will also be asked that will probably be answered truthfully, are not stressful, and act as buffers. Do you reside in Maryland? or Do they call you Jim? are examples of irrelevant questions.
If the police have facts about a crime that have not become public or common knowledge (facts that would be known to the guilty subject but not to the innocent), they will use a concealed knowledge test. In another version of the concealed knowledge test, the examiner does not know the critical item but believes the examinee does know.
A relevant-irrelevant test differs from a control question test in several ways. It has few, if any, control questions on each chart, the sequence of questions usually varies from chart to chart, and the amplitude of reactions to relevant questions is not compared with the amplitude of reactions to the control questions. This type of test is widely used for multiple-issue testing, such as that used for commercial and counterintelligence screening.
Regardless of the test format used, three physiological measurements are normally recorded:
1. Volumetric measures taken from the upper arm: A standard blood pressure cuff is placed on the arm over the brachial artery and inflated to about 60 mm Hg pressure for an indirect measure of blood pressure variables, together with the strength and rate of pulsation from the heart. PA1 2. Respiratory measures taken from expansion and contraction of the thoracic and abdominal areas using rubber tubes placed around the subject: The resulting data are closely related to the amount of gaseous exchange. PA1 3. Skin conductivity (or resistance) measures of electrodermal activity, largely influenced by eccrine (sweat) gland activity: Electrodes are attached to two fingers of the same hand and a galvanometer records the measured skin conductance or resistance to an electrical current.
When some charts are scored, the examiner cannot make a clear decision and must score the chart as inconclusive. From analyzing charts where decisions were made, a government agency completed a report on polygraph validity based on all the studies of real cases conducted since 1980. Examiner decisions were compared with other results such as confessions, evidence, and judicial disposition. Ten studies, which considered the outcome of 2,042 cases, were reviewed.
It must be pointed out, however, that studies of real polygraph tests are necessarily flawed by the fact that the guilt or innocence of the subject must be determined, and correct calls are more easily confirmed than incorrect calls. For example, if the test shows that the subject is guilty, the examiner will often obtain a confession. Thus, tests scored as guilty are more often confirmed if the subject is guilty.
If the test shows that the subject is innocent, other people may be investigated. If another person is found to be guilty, the test becomes confirmed innocent.
With this in mind, and assuming that every disagreement was a polygraph error, the results in the report indicate an accuracy (or validity) of 98% for the 2,042 confirmed cases. For deceptive cases, the accuracy was also 98%, and for nondeceptive cases, 97%.
Mock-crime studies generally have correct calls about 85% of the time. Because of the nature of mock-crime studies, it is believed that real-crime tests are scored as accurately or more accurately.
The accuracy of polygraph decisions for real cases, then, is somewhere between the 85% demonstrated with mock-crime studies and the 98% demonstrated with confirmed charts.
In 1973, the concept of quantifying polygraph patterns for computer analysis was first presented. Later, analysts at the psychology laboratory at the University of Utah began to develop a computerized scoring algorithm, employing a few of the many variables available in physiological patterns. Their research suggested that the most useful measures were the amplitude and duration of the electrodermal response, the rise and fall of the cardiovascular pattern (related to blood pressure changes), and the length of the respiration tracing within a fixed time sequence. These responses were incorporated into a special-purpose computer analytic system, marketed under the name CAPS (Computer Assisted Polygraph System).
A novel aspect of the CAPS system was the introduction of decisions based on a probability figure. For example, deception might be indicated with a probability of 0.89. The probabilities were developed by using both laboratory and field polygraph data, the latter being tests conducted by the U.S. Secret Service that were confirmed by confession. Two other features of the CAPS system are its ability to rank-order reactions and its analytic system, which gives the greatest weight to electrodermal responses, less to respiratory responses, and the least to cardiovascular responses. Before this work, scoring systems gave equal weight to responses from each of the three physiological recordings.
A deficiency of the CAPS system is that the data are taken from a field polygraph instrument that is often nonlinear, and the analog-to-digital conversion (ADC) is performed after some processing. New instruments will reduce distortions in the data by performing the ADC before any processing, displaying, or printing.
In 1989, Axciton Systems, Inc. of Houston, Tex. developed a new commercial computerized polygraph. This system features a computer that processes the physiological signals directly, scrolls the physiological data across a screen in real time during testing, provides for a later printout, records the test on a hard drive or a floppy disk, and provides a system for ranking subject responses.
The system has been field tested with real cases in a Texas police department and is user friendly. The charts, printed after the test, look like standard polygraph charts and can be hand-scored by traditional methods.
The availability of the Axciton system and the probability of other new computerized polygraphs becoming available requires that new, more accurate algorithms be developed for incorporation in these new systems.